There is little use of your system gcc (only used to build the tools to compress the filesystem and create the final image), you need a supplied cross-compiler - but huge dependence on system "make", path and "CFLAG" variables in my experience (not on WRT150n I must add)!

Try a firmware such as WAG200G/WRT54GL - they have pre-built toolchain included in the distro.

I just checked, the WRT150 download includes the pre-built toolchain looks very similar to the WRT54GL range, you probably just have to copy the contents of the tools directory to /opt and add them to your path as per the first couple of steps of stage 2 in Thibor's instruction to build his firmware:-